Radio communication system, base station apparatus, radio terminal, and communication control method

ABSTRACT

A radio communication system includes a first base station ( 11 ) that manages a first cell ( 110 ), a second base station ( 12 ) that manages a second cell ( 120 ), and a radio terminal ( 2 ). The radio terminal ( 2 ) supports dual connectivity involving a bearer split in which a first network bearer between the radio terminal ( 2 ) and a core network ( 3 ) is split over the first base station ( 11 ) and the second base station ( 12 ). The first base station ( 11 ) receives, from the second base station ( 12 ), bearer split status information about communication of the first network bearer in the second base station ( 12 ), and performs control of an access stratum related to the first network bearer. It is thus possible to contribute, for example, to an improvement in control of an access stratum when dual connectivity involving a bearer split is performed.

TECHNICAL FIELD

This application relates to a radio communication system in which basestations communicate with the same radio terminal in their respectivecells.

BACKGROUND ART

To improve deterioration in communication quality due to the recentrapid increase in mobile traffic and to achieve higher-speedcommunication, 3GPP Long Term Evolution (LTE) specifies a carrieraggregation (CA) function to allow a radio base station (eNode B (eNB))and a radio terminal (User Equipment (UE)) to communicate with eachother using a plurality of cells. The cells which can be used by the UEin the CA are limited to cells of one eNB (i.e., cells that are servedor managed by the eNB). The cells used by the UE in the CA areclassified into a primary cell (PCell) that is already used as a servingcell when the CA is started and a secondary cell(s) (SCell(s)) that isused additionally or subordinately. In the PCell, Non Access Stratum(NAS) mobility information (NAS mobility information) and securityinformation (security input) is sent and received during radioconnection (re)-establishment (RRC Connection Establishment, RRCConnection Re-establishment) (see Section 7.5 in Non-Patent Literature1).

In the CA, SCell configuration information transmitted from the eNB tothe UE includes SCell radio resource configuration information common toUEs (RadioResourceConfigCommonSCell) and SCell radio resourceconfiguration information dedicated to a specific UE(RadioResourceConfigDedicatedSCell). The latter information mainlyindicates a dedicated configuration (PhysicalConfigDedicated) for aphysical layer. When cells (carriers) having different transmissiontimings (Timing Advance: TA) are aggregated in an uplink, configurationinformation (MAC-MainConfigSCell) about a Medium Access Control (MAC)sublayer is also transmitted from the eNB to the UE. However, theconfiguration information about the MAC sublayer includes only anSTAG-Id, which is an index of TA Group (TAG) representing a set of cellsincluded in the same TA (see Section 5.3.10.4 in Non-Patent Literature2). The other configurations for the MAC sublayer in the SCell are thesame as those in the PCell.

One of the ongoing study items in the LTE standardization related mainlyto a Heterogeneous Network (HetNet) environment is dual connectivity inwhich the UE performs communication using a plurality of cells of aplurality of eNBs (see Non Patent-Literature 3). Dual connectivity is aprocess to allow an UE to perform communication simultaneously usingboth radio resources (i.e., cells or carriers) provided (or managed) bya main base station (master base station, Master eNB (MeNB)) and a subbase station (secondary base station, Secondary eNB (SeNB)). Dualconnectivity enables inter-eNB CA in which the UE aggregates a pluralityof cells managed by different eNBs. Since the UE aggregates radioresources managed by different nodes, dual connectivity is also called“inter-node radio resource aggregation”. The MeNB is connected to theSeNB through an inter-base-station interface called Xn. The MeNBmaintains, for the UE in dual connectivity, the connection (S1-MME) to amobility management apparatus (Mobility Management Entity (MME)) in acore network (Evolved Packet Core (EPC)). Accordingly, the MeNB can becalled a mobility management point (or mobility anchor) of the UE. Forexample, the MeNB is a Macro eNB, and the SeNB is a Pico eNB or LowPower Node (LPN).

Further, in dual connectivity, a bearer split for splitting a networkbearer (EPS bearer) over the MeNB and the SeNB has been studied. Theterm “network bearer (EPS Bearer)” used in this specification means avirtual connection that is configured between a UE and an endpoint(i.e., Packet Data Network Gateway (P-GW)) in a core network (EPC) foreach service provided to the UE. In an alternative of the bearer split,for example, both a radio bearer (RB) in a cell of the MeNB and a radiobearer in a cell of the SeNB are mapped to one network bearer. The radiobearer (RB) described herein refers mainly to a data radio bearer (DRB).The bearer split will contribute to a further improvement in userthroughput.

CITATION LIST Non Patent Literature

-   [Non-Patent Literature 1] 3GPP TS 36.300 V11.5.0 (2013 March), “3rd    Generation Partnership Project; Technical Specification Group Radio    Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA)    and Evolved Universal Terrestrial Radio Access Network (E-UTRAN);    Overall description; Stage 2 (Release 11)”, March, 2013-   [Non-Patent Literature 2] 3GPP TS 36.331 V11.4.0 (2013 June), “3rd    Generation Partnership Project; Technical Specification Group Radio    Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA);    Radio Resource Control (RRC); Protocol specification (Release 11)”,    June, 2013-   [Non-Patent Literature 3] 3GPP TR 36.842 V0.2.0 (2013 May), “3rd    Generation Partnership Project; Technical Specification Group Radio    Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA);    Study on Small Cell Enhancements for E-UTRA and E-UTRAN—Higher layer    aspects (Release 12)”, May, 2013

SUMMARY OF INVENTION Technical Problem

In the LTE, an UE generates an uplink (UL) Medium Access ControlProtocol Data Unit (MAC PDU) to be transmitted using available resources(Uplink Grant) allocated from an eNB. One MAC PDU is also called atransport block. In the generation of an UL MAC PDU, logical channelsconfigured in the UE are multiplexed on one MAC PDU. At this time, it isnecessary to guarantee the QoS of each EPS bearer configured in theuplink. Accordingly, the UE generates an UL MAC PDU in accordance with aLogical Channel Prioritization (LCP) procedure. In the Logical ChannelPrioritization (LCP) procedure, a priority and a Prioritized Bit Rate(PBR) are given to each logical channel. The PBR is a bit rate which isprovided to one logical channel before allocating any resource to alogical channel having a lower priority. The PBR is configured by theeNB for each logical channel. In the LCP procedure, first, all logicalchannels are guaranteed to be allocated resources corresponding torespective PBRs in descending order of their priorities. Next, if thereare still any available resources left after all logical channels havebeen served up to their PBR, the remaining resources are allocated tothe logical channels in a descending order of priorities of the logicalchannels until there is no data of logical channels, or until theallocated resources are used up.

However, in dual connectivity involving a bearer split, it is consideredthat the MeNB and the SeNB each independently perform Radio ResourceManagement (RRM). Accordingly, there is a possibility that the MeNB andthe SeNB each independently perform the above-mentioned LCP procedure,which may lead to an unfairness between resources (i.e., effective bitrate) allocated to a logical channel (or EPS bearer, radio bearer) whichis not subjected to a bearer split and is transmitted only in the PCelland resources allocated to a logical channel (or EPS bearer, radiobearer) which is subjected to a bearer split and is transmitted in thePCell and the SCell. In other words, the balance of resource allocationbetween a logical channel which is not subjected to a bearer split and alogical channel which is subjected to a bearer split may be lost, andconsequently, the LCP procedure may not function as intended.

In the case of performing dual connectivity involving a bearer split,there is a possibility that expected performance cannot be obtained notonly in the generation of MAC PDUs (i.e., the LCP procedure) describedabove, but also in other Layer 1/Layer 2 control in an access stratum.For example, in uplink transmission power control (PC), there is apossibility that the distribution of transmission power between theuplink transmission in the PCell and the uplink transmission in theSCell may not be performed as intended. Further, in the case ofperforming dual connectivity involving a bearer split, there is apossibility that expected performance cannot be obtained not only in theuplink Layer 1/Layer 2 control, but also in the downlink Layer 1/Layer 2control. It is also possible that expected performance cannot beobtained in control of layer 3 of the Access stratum (i.e., RadioResource Control (RRC)) in the uplink or the downlink or both.

Accordingly, one object to be achieved by embodiments disclosed in thespecification is to contribute to an improvement in control of an accessstratum when dual connectivity involving a bearer split is performed.Other objects and novel features will become apparent from the followingdescription and the accompanying drawings.

Solution to Problem

In an embodiment, a radio communication system includes a first basestation that manages a first cell, a second base station that manages asecond cell, and a radio terminal. The radio terminal supports dualconnectivity involving a bearer split in which a first network bearerbetween the radio terminal and a core network is split over the firstbase station and the second base station. The first base station isconfigured to receive, from the second base station, bearer split statusinformation about communication of the first network bearer in thesecond base station, and to perform control of an access stratum relatedto the first network bearer.

In an embodiment, a base station apparatus includes a communicationcontrol unit configured to control dual connectivity involving a bearersplit in which a first network bearer between a radio terminal and acore network is split over the base station apparatus and a neighborbase station. The communication control unit is configured to receive,from the neighbor base station, bearer split status information aboutcommunication of the first network bearer in the neighbor base station,and to perform control of an access stratum related to the first networkbearer.

In an embodiment, a base station apparatus includes a communicationcontrol unit configured to control dual connectivity involving a bearersplit in which a first network bearer between a radio terminal and acore network is split over the base station apparatus and a neighborbase station. The communication control unit is configured to transmit,to the neighbor base station, bearer split status information aboutcommunication of the first network bearer in the base station apparatus.The bearer split status information triggers the neighbor base stationto perform control of an access stratum related to the first networkbearer.

In an embodiment, a radio terminal is used in the radio communicationsystem described above and includes a communication control unitconfigured to control dual connectivity involving a bearer split inwhich the first network bearer is split over first and second basestations. The communication control unit is configured to performcontrol of an access stratum related to the first network bearer basedon an instruction from the first base station.

In an embodiment, a control method includes: (a) starting, by a firstbase station, communication of dual connectivity involving a bearersplit in which a first network bearer between a radio terminal and acore network is split over the first base station and a second basestation; and (b) receiving, by the first base station from the secondbase station, bearer split status information about communication of thefirst network bearer in the second base station, and performing, by thefirst base station, control of an access stratum related to the firstnetwork bearer.

In an embodiment, a control method includes: (a) starting, by a secondbase station, communication of dual connectivity involving a bearersplit in which a first network bearer between a radio terminal and acore network is split over a first base station and the second basestation; and (b) transmitting, to the first base station, bearer splitstatus information about communication of the first network bearer inthe second base station. The bearer split status information triggersthe first base station to perform control of an access stratum relatedto the first network bearer.

In an embodiment, a program includes instructions (software codes) forcausing a computer to perform the above-described method when theprogram is loaded into the computer.

Advantageous Effects of Invention

According to the embodiments described above, it is possible tocontribute to an improvement in control of an access stratum when dualconnectivity involving a bearer split is performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram showing an example of a user plane protocol stackin a downlink direction of LTE Layer 2 related to dual connectivityinvolving a bearer split;

FIG. 1B is a diagram showing another example of the user plane protocolstack in the downlink direction of LTE Layer 2 related to dualconnectivity involving a bearer split;

FIG. 2A is a diagram showing an example of a user plane protocol stackin an uplink direction of LTE Layer 2 related to dual connectivityinvolving a bearer split;

FIG. 2B is a diagram showing another example of the user plane protocolstack in the uplink direction of LTE Layer 2 related to dualconnectivity involving a bearer split;

FIG. 3 is a diagram showing a configuration example of a radiocommunication system according to first to fourth embodiments;

FIG. 4 is a diagram showing a LogicalChannelConfig information elementspecified in 3GPP TS 36.331;

FIG. 5A is a diagram showing UplinkPowerControl information elementsspecified in 3GPP TS 36.331;

FIG. 5B is a diagram showing UplinkPowerControl information elementsspecified in 3GPP TS 36.331;

FIG. 6 is a sequence diagram showing an example of a control procedureregarding dual connectivity involving a bearer split according to thefirst embodiment;

FIG. 7 is a sequence diagram showing an example of a control procedureregarding dual connectivity involving a bearer split according to thesecond embodiment;

FIG. 8A is a schematic diagram showing an example of generating anuplink MAC PDU when no bearer split is performed;

FIG. 8B is a schematic diagram showing an example of generating uplinkMAC PDUs when bearer split is performed;

FIG. 8C is a schematic diagram showing an example of generating uplinkMAC PDUs when bearer split is performed;

FIG. 9 is a sequence diagram showing an example of a control procedureregarding dual connectivity involving a bearer split according to thethird embodiment;

FIG. 10 is a block diagram showing a configuration example of an MeNBaccording to the first to fourth embodiments;

FIG. 11 is a block diagram showing a configuration example of an SeNBaccording to the first to fourth embodiments; and

FIG. 12 is a block diagram showing a configuration example of a UEaccording to the first to fourth embodiments.

DESCRIPTION OF EMBODIMENTS

Specific embodiments will hereinafter be described in detail withreference to the drawings. The same or corresponding elements aredenoted by the same reference symbols throughout the drawings, andrepeated descriptions thereof are omitted as appropriate for clarity ofthe explanation.

First Embodiment

First, with regard to some embodiments including this exemplaryembodiment, several examples of dual connectivity (e.g., inter-noderadio resource aggregation) involving a bearer split are described.FIGS. 1A and 1B show two alternatives of a user plane protocol stack ina downlink direction of LTE Layer 2 related to dual connectivityinvolving a bearer split. In the bearer split, a network bearer (EPSbearer) configured between a UE and an endpoint (i.e., P-GW) of a corenetwork (EPC) is split over an MeNB 11 and an SeNB 12. In thealternatives shown in FIGS. 1A and 1B, an EPS bearer #2 is split overthe MeNB 11 and the SeNB 12. An EPS bearer #1 shown in FIGS. 1A and 1Bis a normal bearer which is not subjected to a bearer split.Accordingly, the EPS bearer #1 is mapped in a one-to-one correspondenceto the radio bearer in a cell of the MeNB 11.

In the alternatives shown in FIGS. 1A and 1B, one data radio bearer(DRB), which has a one-to-one association with the EPS bearer #,2 issplit over the MeNB 11 and the SeNB 12 in a Packet Data ConvergenceProtocol (PDCP) sublayer, a Radio Link Control (RLC) sublayer, or a MACsublayer of Layer-2. Specifically, in the alternative shown in FIG. 1A,a PDCP entity of the MeNB 11 terminates the S1-U of the EPS bearer #2.In other words, one S1 bearer and one data radio bearer (DRB) which aremapped to the EPS bearer #2 are terminated at the PDCP sublayer of theMeNB 11. Further, in the alternative shown in FIG. 1A, the MeNB 11 andthe SeNB 12 have independent RLC entities for bearer split, and one DRB(or PDCP bearer) terminated at the MeNB 11 is split into the RLC bearerof the MeNB 11 and the RLC bearer of the SeNB 12. Note that, the term“PDCP bearer” means a connection terminated at the PDCP sublayers of theeNB and the UE. The PDCP bearer can also be called a PDCP Protocol DataUnit (PDCP PDU). In the example shown in FIG. 1A, there is one PDCPbearer related to the EPS bearer #2 to be split, and this PDCP bearer isterminated at the MeNB 11 and the UE 2. On the other hand, the term “RLCbearer” means a connection terminated at the RLC sublayers of the eNBand the UE. The RLC bearer can also be called an RLC PDU or a logicalchannel. In the example shown in FIG. 1, there are two independent RLCbearers associated with the EPS bearer #2. One of the two RLC bearers isterminated at the MeNB 11 and the UE 2, and the other one is terminatedat the SeNB 12 and the UE 2. Accordingly, in the architecture shown inFIG. 1A, the UE 2 is required to have two independent RLC entitiesassociated with the EPS bearer #2 to be split.

Like in the alternative shown in FIG. 1A, in the alternative shown inFIG. 1B, a PDCP entity of the MeNB 11 terminates the S1-U of the EPSbearer #2. Further, as for the EPS bearer #2 to be split, the MeNB 11has a master RLC entity and the SeNB 12 has a slave RLC entity. In thealternative shown in FIG. 1B, the UE 2 is required to have only one RLCentity associated with the EPS bearer #2 to be split. In the downlink,the slave RLC entity of the SeNB 12 receives, from the master RLC entityof the MeNB 11, RLC PDUs that has already been generated by the masterRLC entity and allocated to the slave RLC for transmission.

The following description is based on an assumption that a cell of theMeNB 11 can be called a PCell and a cell of the SeNB 12 can be called anSCell from the viewpoint of the conventional Carrier Aggregation (CA).However, the scope of this embodiment is not limited to this. Forexample, when the radio terminal (UE) performs the CA (Intra-SeNB CA) ona plurality of cells of the SeNB 12 (i.e., at least a plurality ofdownlink Component Carriers (CCs)) during dual connectivity, one of thecells of the SeNB 12 subjected to the CA may be defined as a PCell or apseudo PCell which functions similarly to a PCell. The pseudo PCell canalso be called an Anchor cell, a Master cell, a Control cell, or thelike. In the CA of the cells of the SeNB 12, the former cell (the PCellof the SeNB 12) has a role similar to that of the PCell in theconventional CA. In the PCell of the SeNB 12, for example, the eNB(SeNB) carries out SCell configuration or SCell activation/deactivationfor the CA, and the UE carries out Radio Link Monitoring (RLM)/RadioLink Failure (RLF) detection. Further, the UE may perform, for example,transmission of L1/L2 control information (e.g., CQI, CSI, HARQfeedback, Scheduling Request) in an uplink control channel (PUCCH),transmission of (a preamble of) a Contention-based Random Access Channel(RACH), and reception of a response (Random Access Response (RAR)) tothe RACH Preamble. The latter cell (the Pseudo PCell of the SeNB 12) hasa role as a cell having a PCell function regarding the control of a UserPlane (UP) in the conventional CA. In the Pseudo PCell of the SeNB 12,the UE may perform, for example, transmission of L1/L2 controlinformation in the uplink control channel (PUCCH), transmission of (apreamble of) a Contention-based RACH, and reception of a response (RAR)to the RACH Preamble. Furthermore, in the UE, the cells of the MeNB 11and the cells of the SeNB 12 need not necessarily have a hierarchicalrelationship (PCell and SCell) or a master-slave relationship.

The user plane protocol stack for dual connectivity involving a bearersplit is not limited to the alternatives shown in FIGS. 1A and 1B. Inthe bearer split, for example, two radio bearers may be mapped to onenetwork bearer (EPS bearer). When the terms in FIGS. 1A and 1B are used,it can be expressed that the EPS bearer #2 is mapped to both the radiobearer (RB) in the cell (PCell) of the MeNB 11 and the radio bearer inthe cell (SCell) of the SeNB 12. For convenience of explanation, theradio bearer in the cell (PCell) of the MeNB 11 is defined herein as aPrimary RB (P-RB) and the radio bearer (RB) in the cell (SCell) of theSeNB is defined herein as a Secondary RB (S-RB). Since the bearer splitis mainly applied to data radio bearers (DRBs), the P-RB and the S-RBcan also be called P-DRB and S-DRB, respectively. For example, the MeNB11 may terminate the S1-U of the EPS bearer #2, and the MeNB 11 and theSeNB 12 may have independent PDCP entities. Further, in a new layerhigher than the PDCP entity of the MeNB 11, a downlink S1-U packetstream of the EPS bearer #2 may be split over the PDCP entity of theMeNB 11 and the PDCP entity of the SeNB 12. In this case, there are twoindependent PDCP bearers related to the EPS bearer #2. One of the twoPDCP bearers is terminated at the MeNB 11 and the UE 2, and the otherone is terminated at the SeNB 12 and the UE 2.

The user plane protocol stack in the uplink direction of LTE Layer 2related to dual connectivity involving a bearer split is similar to thatin the downlink direction described above. FIGS. 2A and 2B show twoalternatives of the user plane protocol stack in the uplink direction ofthe UE 2, and correspond to FIG. 1A and FIG. 1B, respectively. In thealternative shown in FIG. 2A, one PDCP entity of the UE 2 receives userdata of the EPS bearer #2 from an upper layer. The PDCP entity of the UE2 distributes PDCP PDUs between a MAC entity to transmit to the MeNB 11and a MAC entity to transmit to the SeNB 12, and sends the MAC entities.In other words, the PDCP PDUs (i.e., PDCP bearer) are split over an RLCbearer to be transmitted to the MeNB 11 and an RLC bearer to betransmitted to the SeNB 12. Like in the alternative shown in FIG. 1B, inthe alternative shown in FIG. 2B, the UE 2 has a master RLC entity (RLCentity for the MeNB 11 shown in the left side of FIG. 2B) and a slaveRLC entity (RLC entity for the SeNB 12 shown in the right side of FIG.2B). The slave RLC entity of the UE 2 receives, from the master RLCentity, the RLC PDUs which are already generated by the master RLCentity and allocated to the slave RLC for transmission. The alternativesshown in FIGS. 2A and 2B are only illustrative and other architecturescan also be employed. For example, in the alternatives shown in FIG. 2Aand FIG. 2B, the UE 2 has the MAC entity for the MeNB 11 and the MACentity for the SeNB 12. However, the UE 2 may have only one MAC entityfor uplink transmission.

FIG. 3 shows a configuration example of a radio communication systemaccording to some embodiments including this embodiment. The radiocommunication system includes a radio access network (RAN) 1, a radioterminal (UE) 2, and a core network 3. In the EPS, the RAN 1 is anEvolved UMTS Terrestrial Radio Access Network (E-UTRAN), and the corenetwork 3 is an Evolved Packet Core (EPC). The E-UTRAN 1 includes basestations (evolved NodeBs (eNBs)) 11 and 12. The eNB 11 manages a cell110, and the eNB 12 manages a cell 120. The UE 2 is connected to theeNBs 11 and 12 by means of a radio access technology. The EPC 3 isaccessed from the UE 2 through the E-UTRAN 1, and provides the UE 2 witha connection service (e.g., Internet Protocol (IP) connection service)for connecting to an external network (Packet Data Network (PDN)). Inaddition, FIG. 3 shows a HetNet environment. Specifically, the cell 110shown in FIG. 3 has a coverage area larger than that of the cell 120.FIG. 3 also shows a hierarchical cell configuration in which the cell120 is located within the cell 110. However, the cell configurationshown in FIG. 3 is merely an example. For example, the cells 110 and 120may have the same degree of coverage. In other words, the radiocommunication system according to this embodiment may be applied to ahomogeneous network environment.

The E-UTRAN 1 and the UE 2 according to this embodiment support dualconnectivity involving a bearer split. Specifically, while using thecell 110 of the eNB (i.e., MeNB) 11 as a primary cell (PCell), the UE 2can use the cell 120 of the eNB (i.e., SeNB) 12 as a secondary cell(SCell). The UE 2 can receive and/or transmit data of one EPS bearersubjected to a bearer split through the PCell 110 and the SCell 120.

In order to improve the Layer 1/Layer 2 control of an access stratum inthe case of performing dual connectivity involving a bearer split, theMeNB 11 and the SeNB 12 according to this embodiment carry out a controlprocedure or signalling as described below. The SeNB 12 is configured totransmit, to the MeNB 11, bearer split status information aboutcommunication in the SeNB 12 (i.e., SCell 120) of the EPS bearer to besubjected to a bearer split (hereinafter referred to as a split EPSbearer). The MeNB 11 is configured to perform control of the accessstratum related to the split EPS bearer in response to receiving thebearer split status information from the SeNB 12.

The bearer split status information may include, for example, at leastone of communication status information, radio resource controlinformation, and admission control information.

The communication status information indicates a communication status ofthe split EPS bearer in the SeNB 12 (i.e., SCell 120). The communicationstatus of the split EPS bearer in the SeNB 12, which is indicated by thecommunication status information and sent to the MeNB 11 from the SeNB12, may be a communication status in Layer 1 or Layer 2 of the SCell120. More specifically, the communication status of the split EPS bearerin the SeNB 12 may include at least one of the following items (1) to(6):

(1) Statistics of throughput;(2) Statistics of allocated radio resources;(3) Statistics of packet losses;(4) Statistics of power headroom;(5) Information about retransmission control in the Radio Link Control(RLC) sublayer; and(6) Information about packet discarding in the Radio Link Control (RLC)sublayer.

The statistics of throughput may be, for example, at least one of anaverage value, a minimum value, and a maximum value of a data rate(e.g., transmission rate or data rate of PDCP SDU, PDCP PDU, RLC PDU, orMAC PDU (i.e., Transport Block)) of the UE 2 in the SeNB 12. Thestatistics of allocated radio resources may be, for example, at leastone of an average value, a minimum value, and a maximum value of radioresources allocated to the UE 2 in the SeNB 12. In this case, the radioresources may be, for example, resource blocks. When the SeNB 12transmits data of the split EPS bearer to the UE 2 by using a pluralityof cells, the statistics of throughput and the statistics of radioresources may be a value in each of the plurality of cells, or the totalvalue of the plurality of cells.

The statistics of packet losses may be, for example, the number or ratioof discarded packets in a radio interface (LTE-Uu interface) between theSeNB 12 and the UE 2, or in an inter-base-station interface (Xninterface) between the MeNB 11 and the SeNB 12. In this case, thepackets may be, for example, PDCP SDUs, PDCP PDUs, RLC PDUs, or MAC PDUs(i.e., Transport Blocks). The statistics of packet losses may bestatistics observed not for the Xn interface, but for an X2 interface oran S1 interface.

The statistics of uplink power headroom indicate, for example, anaverage value of power headroom of the UE 2 for the SCell 120 (in apredetermined period). The power headroom indicates a difference (i.e.,surplus transmission power) between the uplink maximum transmissionpower of the UE 2 and the transmission power of a Physical Uplink Sharedchannel (PUSCH) in the present subframe. The UE 2 reports the powerheadroom for the SCell 120 to the SeNB 12. The UE 2 may report the powerheadroom for the PCell 110 and the power headroom for the SCell 120 tothe SeNB 12.

The information about retransmission control in the RLC sublayer mayindicate a NACK ratio of Automatic Repeat Request (ARQ) for RLC PDUs(i.e., logical channel) of the split EPS bearer (i.e., a ratio of NACKswith respect to the total of ACKs and NACKs), the number ofretransmissions in the ARQ, or a frequency of occurrence ofretransmission in the ARQ.

The information about packet discarding in the RLC sublayer may indicatethe rate or number of discarded RLC SDUs of the split EPS bearer, or thedata amount of discarded RLC SDUs. Packet discarding in the RLC sublayer(i.e., discarding of RLC SDUs) may be executed in response to aninstruction from the PDCP sublayer of the MeNB 11. Alternatively, theRLC sublayer of the SeNB 12 may independently determine whether toperform packet discarding.

The communication status information transmitted from the SeNB 12 to theMeNB 11 may indicate, for example, a communication status monitored foreach split EPS bearer, monitored for each ratio bearer mapped to thesplit EPS bearer, monitored for each SCell 120, or monitored for eachSeNB 12. The communication status monitored for each SCell 120 may beobtained by observation of each SCell 120 and each radio terminal (UE)which performs a bearer split, or may be obtained by observation of eachSCell 120 and a plurality of radio terminals which perform a bearersplit in the SeNB 12. The same is true of the communication statusmonitored for each SeNB 12.

Next, the control of the access stratum performed by the MeNB 11 isdescribed. The control of the access stratum may be, for example, aLayer 1 control, a Layer 2 control, a Layer 3 control, or anycombination thereof. Several examples of the Layer 1/Layer 2 control ofthe access stratum are given below. Note that, the Layer 1/Layer 2control of the access stratum may be a Layer 3 (RRC) control orsignalling regarding functions in Layer 1 (PHY)/Layer 2 (MAC, RLC, andPDCP). For example, the MeNB 11 may perform at least one of thefollowing controls (a) to (c) in response to receiving from the SeNB 12the communication status of the split EPS bearer in the SeNB 12.

(a) Control for Generation of Uplink (UL) MAC PDUs

Even during execution of the bearer split, the UE 2 should generate MACPDUs in consideration of an EPS bearer QoS (QoS class identifier (QCI),a guaranteed bit rate (GBR), an aggregate maximum bit rate (AMBR), etc.)for each of all EPS bearers including a split EPS bearer and a non-splitEPS bearer. Accordingly, if the uplink LCP procedure does not functionas intended due to, for example, the excess uplink throughput of thesplit EPS bearer in the SCell 120, the MeNB 11 may adjust an uplinkPrioritized Bit Rate (PBR) or Bucket Size Duration (BSD) or both ofthem, which are applied to the split EPS bearer or the non-split EPSbearer or both of them, so that the LCP procedure functions as intended.For example, when the throughput of the split EPS bearer in the SCell120 is excessive, the MeNB 11 may decrease the uplink PBR applied to thesplit EPS bearer in the PCell 110 and may increase the uplink PBRapplied to the non-split EPS bearer in the PCell 110. In this case, thePBR can also be called a prioritized resource amount. For example, theRRC layer in the MeNB 11 may determine the PBR applied to the split EPSbearer and the PBR applied to the non-split EPS bearer, and may notifythe UE 2 of the determined PBR values by RRC signalling. The PBR appliedto the split EPS bearer in the PCell 110 may be the same as or differentfrom that in the SCell 120. Further, the MeNB 11 may determine the BSDfor each of the split EPS bearer and the non-split EPS bearer, and maynotify the UE 2 of the obtained BSD values. The BSD of the split EPSbearer in the PCell 110 may be the same as or different from that in theSCell 120. Non-Patent Literature 2 (3GPP TS 36.331) specifies parametersregarding the LCP including the PBR and the BSD (see FIG. 4). Theparameters shown in FIG. 4, including the PBR and the BSD, may beadjusted in the control of the access stratum according to thisembodiment. The parameters shown in FIG. 4 may be set separately for thesplit EPS bearer and the non-split EPS bearer. The parameters set forthe split EPS bearer in the cell 110 of the MeNB 11 may be the same ordifferent from the parameters set for the split EPS bearer in the cell120 of the SeNB 12.

(b) Uplink (UL) Transmission Power Control

The MeNB 11 may adjust the transmission power of the UE 2 to achieveintended distribution of transmission power between uplink transmissionin the PCell 110 and uplink transmission in the SCell 120. For example,in response to determining, based on the communication statusinformation received from the SeNB 12, that the power headroom of the UE2 in the PCell 110 is less than the power headroom of the UE 2 in theSCell 120 by more than a predetermined amount, the MeNB 11 may adjust aparameter(s) used for a formula for calculating P_(CMAX) so as toincrease the configured maximum transmission power P_(CMAX, PCELL) inthe PCell 110 of the UE 2 and to decrease the configured maximumtransmission power P_(CMAX, SCELL) in the SCell 120 of the UE 2. Theformula for calculating P_(CMAX) is specified in 3GPP TS 36.301.Specifically, the MeNB 11 may adjust the maximum transmission power(i.e., transmit power limit) P_(EMAX, PCELL), which is allowed for theUE 2 in the PCell 110, or the maximum transmission powerP_(EMAX, SCELL), which is allowed for the UE 2 in the SCell 120, or bothof them. Non-Patent Literature 2 (3GPP TS 36.331) specifies parametersregarding the UL transmission power control (see FIGS. 5A and 5B). Theparameters shown in FIGS. 5A and 5B may be adjusted in the control ofthe access stratum according to this embodiment. The parameters set tothe cell 110 of the MeNB 11 may be the same or different from theparameters set to the cell 120 of the SeNB 12.

(c) Control for Generation of Downlink (DL) MAC PDUs

The MeNB 11 may perform control regarding the downlink similar to theabove-described control for generation of uplink MAC PDUs. Specifically,in response to determining, based on the communication statusinformation received from the SeNB 12, that the downlink LCP proceduredoes not function as intended, the MeNB 11 may adjust the downlink PBRfor the split EPS bearer or the downlink PBR for the non-split EPSbearer or both of them, so that the LCP procedure functions as intended.For example, when the downlink throughput of the split EPS bearer in theSCell 120 is excessive, the MeNB 11 may decrease the downlink PBRapplied to the split EPS bearer in the PCell 110 and may increase thedownlink PBR applied to the non-split EPS bearer in the PCell 110. Inthis case, the PBR can also be called a prioritized resource amount.

The above description concentrates on an example in which the SeNB 12reports to the MeNB 11 the communication status regarding the split EPSbearer in the SeNB 12 (SCell 120) and the MeNB 11 performs the Layer1/Layer 2 control of the access stratum. However, it should be notedthat the roles of the MeNB 11 and the SeNB 12 are interchangeable.Specifically, the MeNB 11 may report to the SeNB 12 the communicationstatus related to the split EPS bearer in the MeNB 11 (PCell 110). TheSeNB 12 may perform the Layer 1/Layer 2 control of the access stratumrelated to the split EPS bearer in response to receiving thecommunication status information from the MeNB 11 (PCell 110).

Next, a specific example of the control procedure according to thisembodiment is described. FIG. 6 is a sequence diagram showing an exampleof the control procedure regarding dual connectivity involving a bearersplit. In step S11, the control procedure for starting the dualconnectivity involving a bearer split is performed among the MeNB 11,the SeNB 12, and the UE 2. Accordingly, in step S12, the UE 2 performsuplink, downlink, or bidirectional communication of the split EPS bearerwith the MeNB 11 and the SeNB 12.

In step S13, the MeNB 11 sends a bearer split status request to the SeNB12. In step S14, in response to receiving the bearer split statusrequest, the SeNB 12 sends a bearer split status response to the MeNB11. The bearer split status response includes the bearer split statusinformation. Note that steps S13 and S14 are only illustrative. Forexample, the SeNB 12 may send the bearer split status informationperiodically or non-periodically, regardless of the request from theMeNB 11.

In step S15, the MeNB 11 performs control (e.g., Layer 1/Layer 2control) of the access stratum related to the split EPS bearer based onthe bearer split status information received from the SeNB 12. Asdescribed above, the MeNB 11 may perform control for generation ofuplink MAC PDUs (e.g., adjustment of PBR), uplink transmission powercontrol (e.g., adjustment of P_(EMAX)), or control for generation ofdownlink MAC PDUs (e.g., adjustment of PBR). In the example shown inFIG. 6, the MeNB 11 transmits to the UE 2 an RRC ConnectionReconfiguration message that contains updated configuration informationabout uplink transmission (updated UL Tx configuration) regarding theLayer 1/Layer 2 control. The UE 2 performs the Layer 1/Layer 2 controlof the access stratum in accordance with the updated configurationinformation received from the MeNB 11. Accordingly, in step S16, the UE2 performs uplink, downlink, or bidirectional communication of the splitEPS bearer with the MeNB 11 and the SeNB 12 in accordance with the Layer1/Layer 2 control by the MeNB 11.

In FIG. 6, the roles of the MeNB 11 and the SeNB 12 are interchangeable.Specifically, the MeNB 11 may send to the SeNB 12 the bearer splitstatus information related to the split EPS bearer in the MeNB 11 (PCell110). Further, the SeNB 12 may perform control of the access stratum inresponse to receiving the bearer split status information from the MeNB11 (PCell 110).

As can be seen from the above description, according to this embodiment,the MeNB 11 (or SeNB 12) is configured to receive the bearer splitstatus information from the SeNB 12 (or MeNB 11) and to perform controlof the access stratum. In some implementations, the bearer split statusinformation includes communication status information indicatingcommunication status of the split EPS bearer in the SeNB 12. In thiscase, according to this embodiment, the Layer 1/Layer 2 control of theaccess stratum is performed based on the communication statusinformation between the MeNB 11 and the SeNB 12. Thus, in thisembodiment, when dual connectivity involving a bearer split isperformed, unfairness between communication of the split EPS bearer andthat of the non-split EPS bearer can be corrected, and generation of MACPDUs, transmission power control, and the like can be optimized so thatthey can be performed as intended.

Second Embodiment

In this embodiment, a specific example of the Layer 1/Layer 2 controlfor uplink transmission, which is included in the control of the accessstratum based on sharing of the bearer split status information betweenthe MeNB 11 and the SeNB 12 according to the first embodiment, isdescribed. A configuration example of a radio communication systemaccording to this embodiment is similar to that shown in FIG. 3.

FIG. 7 is a sequence diagram showing an example of the control procedureregarding dual connectivity (e.g., inter-node radio resourceaggregation) involving a bearer split according to this embodiment. Theprocessing of step S21 may be performed in the same manner as theprocessing of step S11 shown in FIG. 6. Specifically, in step S21, thecontrol procedure for starting dual connectivity involving a bearersplit is performed among the MeNB 11, the SeNB 12, and the UE 2.

In step S22, the MeNB 11 transmits a control message for controllinguplink communication of an EPS bearer(s) that is configured in the UE 2and includes a split EPS bearer. In the example shown in FIG. 7, theMeNB 11 transmits the control message to the UE 2 by using an RRCConnection Reconfiguration message. The control message may include aparameter(s) related to the LCP procedure for generating UL MAC PDUs(e.g., PBR). The control message may also include a control parameter(s)related to the uplink transmission power (e.g., maximum transmissionpower P_(EMAX) allowed for the UE 2 in the PCell 110 or SCell 120).

In step S23, the UE 2 performs uplink communication of a split EPSbearer with the MeNB 11 and the SeNB 12 in accordance with the controlby the MeNB 11 in step S22. Step S23 may include uplink communication ofa non-split EPS bearer in the PCell 110.

The processing of steps S24 and S25 may be performed in the same manneras the processing of steps S13 and S14 shown in FIG. 6. Specifically, instep S24, the MeNB 11 sends a bearer split status request to the SeNB12. In step S25, in response to receiving the bearer split statusrequest, the SeNB 12 sends a bearer split status response including thebearer split status information to the MeNB 11. Instead of performingsteps S24 and S25, the SeNB 12 may send the bearer split statusinformation periodically or non-periodically, regardless of the requestfrom the MeNB 11.

In step S26, the MeNB 11 performs uplink Layer 1/Layer 2 control for thesplit EPS bearer based on the bearer split status information receivedfrom the SeNB 12. In the example shown in FIG. 7, the MeNB 11 transmitsto the UE 2 an RRC Connection Reconfiguration message containing anupdated control message for controlling the uplink communication. Theupdated control message is generated in consideration of the bearersplit status information received from the SeNB 12. For example, theMeNB 11 may update a parameter(s) related to the LCP procedure appliedto generation of uplink MAC PDUs (e.g., PBR), or may update a controlparameter(s) related to the uplink transmission power (e.g., P_(EMAX))so as to correct the unfairness between communication of the split EPSbearer and communication of the non-split EPS bearer.

In step S27, the UE 2 performs uplink communication of the split EPSbearer with the MeNB 11 and the SeNB 12 in accordance with the controlby the MeNB 11 in step S26. Step S26 may include uplink communication ofthe non-split EPS bearer in the PCell 110.

In FIG. 7, the roles of the MeNB 11 and the SeNB 12 are interchangeable.Specifically, the MeNB 11 may report to the SeNB 12 the bearer splitstatus information related to the split EPS bearer in the MeNB 11 (PCell110). Further, the SeNB 12 may perform uplink Layer 1/Layer 2 control inresponse to receiving the bearer split status information from the MeNB11 (PCell 110).

Next, a specific example of the control for generation of uplink MACPDUs is described with reference to FIGS. 8A to 8C. FIG. 8A is aschematic diagram showing an example of generating an uplink MAC PDU inthe MeNB 11 (PCell 110) when no bearer split is performed. FIG. 8A showsan example in which data from two logical channels (i.e., LCH #1 and LCH#2) is multiplexed on available resources (MAC PDU) indicated by anUplink Grant from the MeNB 11. The LCH #1 is assigned a highest priority(first priority) and PBR1. The LCH #2 is assigned a second priority andPBR2. In accordance with the uplink PBR procedure specified in the LTEstandards, resources up to the PBR1 are first allocated to the LCH #1which is of the highest priority, and then resources up to the PBR2 areallocated for the LCH #2. After that, the remaining room in theavailable resources (MAC PDU) is filled with data from the LCH #1 untilthere is no further data from the LCH #1 which is of the highestpriority or there is no further room in the MAC PDU.

FIG. 8B shows a case where a bearer split is performed on the EPS bearercorresponding to the logical channel LCH #2. In the SCell 120, only thelogical channel LCH #2 is configured to the UE 2. Accordingly, uplinkresources of the SCell 120 granted to the UE 2 by the SeNB 12 can beused mainly for transmission of data from the logical channel LCH #2.However, in the example shown in FIG. 8B, the PBRs for the logicalchannels LCH #1 and LCH #2 are the same as those in the example shown inFIG. 8A (i.e., PBR1 and PBR2), and the logical channel LCH #2 of thesplit EPS bearer is provided with the PBR2 in each of the PCell 110 andthe SCell 120. Accordingly, in the example shown in FIG. 8B, the bitrate of the logical channel LCH #2 which is of the second priority ishigher than the bit rate of the logical channel LCH #1 which is of thehighest priority. This state shows a situation where there is unbalancedresource allocation between the logical channel LCH #1 which is notsubjected to a bearer split and the logical channel LCH #2 which issubjected to a bearer split and the LCP procedure does not function asintended.

To overcome the undesirable situation shown in FIG. 8B, the SeNB 12reports to the MeNB 11 the communication status of the logical channelLCH #2 in the SeNB 12 (SCell 120) or the communication status of thesplit EPS bearer (or the radio bearer) associated with the logicalchannel LCH #2. The SeNB 12 may report to the MeNB 11, for example, thethroughput of the logical channel LCH #2 in the SeNB 12 (e.g.,transmission rate or data rate of PDCP SDUs, PDCP PDUs, RLC PDUs, or MACPDUs (i.e., Transport Blocks)). The SeNB 12 may report to the MeNB 11the total uplink throughput of the UE 2 in the SCell 120, instead of thethroughput per logical channel (or EPS bearer, radio bearer). The MeNB11 determines that the throughput of the logical channel LCH #2 relatedto the split EPS bearer is excessive upon considering the PCell 110 andthe SCell 120 as a whole, and thus controls the LCP procedure to correctthe excessive throughput of the logical channel LCH #2. Specifically,the MeNB 11 may increase the PBR1 of the logical channel LCH #1 which isnot subjected to a bearer split, or may decrease the PBR2 of the logicalchannel LCH #2 which is subjected to a bearer split, or may perform bothincrease of the PBR1 and decrease of the PBR2.

FIG. 8C is a schematic diagram showing an example of generating uplinkMAC PDUs after the adjustment of the PBRs. In the example shown in FIG.8C, the PBR1 of the logical channel LCH #1 which is not subjected to abearer split is increased to PBR1′. Further, the PBR2 of the logicalchannel LCH #2 which is subjected to a bearer split is decreased toPBR2′. Accordingly, the bit rate of the logical channel #1 in the PCell110 increases and the bit rate of the logical channel #2 in the PCell110 decreases. Thus, upon viewing the PCell 110 and the SCell 120 as awhole, the balance of resource allocation between the logical channelLCH #1 and the logical channel LCH #2 can be brought closer to theintended state. When only data of a radio bearer (RB) associated withone EPS Bearer is transmitted in the SCell 120, all the availableresources may be simply allocated to the data of the RB, withoutexecuting the LCP algorithm.

The uplink Layer 1/Layer 2 control performed in this embodiment may beuplink transmission power control. In this case, the SeNB 12 may report,to the MeNB 11, information about the power headroom of the UE 2 in theSCell 120 as the communication status information. The information aboutthe power headroom may be statistics, such as an average value of thepower headroom, or other information indicating the size of the powerheadroom. The MeNB 11 may adjust the transmission power of the UE 2 bytaking into account both the power headroom of the UE 2 in the PCell 110and the power headroom of the UE 2 in the SCell 120. For example, asdescribed above, the MeNB 11 may adjust one or both of P_(EMAX,PCELL)and P_(EMAX,SCELL) when it is determined that the power headroom of theUE 2 in the PCell 110 is less than the power headroom of the UE 2 in theSCell 120 by more than a predetermined amount. Specifically, the MeNB 11may increase P_(EMAX,PCELL) and decrease P_(EMAX,SCELL). P_(EMAX,PCELL)represents the maximum transmission power (i.e., transmit power limit)allowed for the UE 2 in the PCell 110, and P_(EMAX,SCELL) represents themaximum transmission power allowed for the UE 2 in the SCell 120.P_(EMAX, PCELL) and P_(EMAX,SCELL) are used to determine the configuredmaximum transmission power P_(CMAX,PCELL) in the PCell 110 andP_(CMAX,SCELL) in the SCell 120. P_(CMAX,PCELL) and P_(CMAX,SCELL) maybe determined according to the calculation formulas (P_(CMAX,C))specified in 3GPP TS 36.301.

Also in the example of the uplink transmission power control, the rolesof the MeNB 11 and the SeNB 12 are interchangeable. Specifically, theMeNB 11 may report, to the SeNB 12, the information about the powerheadroom of the UE 2 in the PCell 110. Further, the SeNB 12 may adjustthe uplink maximum transmission power of the UE 2 in one or both of thePCell 110 and the SCell 120 in consideration of the power headroom ofthe UE 2 in the PCell 110.

Third Embodiment

In this embodiment, a specific example of the Layer 1/Layer 2 controlfor downlink transmission, which is included in the control of theaccess stratum based on sharing of the bearer split status informationbetween the MeNB 11 and the SeNB 12 according to the first embodiment,is described. A configuration example of a radio communication systemaccording to this embodiment is similar to that shown in FIG. 3.

FIG. 9 is a sequence diagram showing an example of the control procedureregarding dual connectivity involving a bearer split according to thisembodiment. The processing of step S31 may be performed in the samemanner as the processing of step S11 shown in FIG. 6. Specifically, instep S31, the control procedure for starting dual connectivity involvinga bearer split is performed among the MeNB 11, the SeNB 12, and the UE2.

In step S32, the MeNB 11 and the SeNB 12 perform downlink communicationof a split EPS bearer with the UE 2. Step S32 may include downlinkcommunication of a non-split EPS bearer in the PCell 110.

The processing of steps S33 and S34 may be performed in the same manneras the processing of steps S13 and S14 shown in FIG. 6. Specifically, instep S33, the MeNB 11 sends a bearer split status request to the SeNB12. In step S34, in response to receiving the bearer split statusrequest, the SeNB 12 sends a bearer split status response including thebearer split status information to the MeNB 11. Instead of performingsteps S33 and S34, the SeNB 12 may send the bearer split statusinformation periodically or non-periodically, regardless of the requestfrom the MeNB 11.

The bearer split status information sent in step S34 may indicateinformation relating to the downlink communication of the UE 2 in theSCell 120, including communication status information, radio resourcecontrol information, or admission control information, or anycombination thereof. The communication status information may indicatestatistics (e.g., average value) of radio resources (i.e., the number ofresource blocks) allocated to the UE 2 in the SCell 120. Thecommunication status information may indicate statistics (e.g., averagevalue) of the throughput (e.g., transmission rate or data rate of PDCPSDUs, PDCP PDUs, RLC PDUs, or MAC PDUs (Transport Blocks)) of the UE 2in the SCell 120. The communication status information may also indicatea packet loss rate, information about retransmission control in the RLCsublayer, information about packet discarding in the RLC sublayer, andthe like.

The radio resource control information may be information about radioresources used in the SCell 120 for data (service) from the split EPSbearer. More specifically, the radio resource control information in theSeNB 12 may include at least one of the following information items (1)to (3):

(1) Information about an increase or decrease in radio resources;(2) Information about available radio resources; and(3) Information about surplus radio resources.

The information about an increase or decrease in radio resources mayindicate, for example, that the number of radio resources can beincreased (or a request to increase the number of radio resources can bemade), or the number of radio resources can be reduced (or a request toreduce the number of radio resources can be made), according to the usestatus or the like of radio resources used for a bearer split (i.e.,split EPB bearer) in the SeNB 12.

The information about available radio resources may indicate, forexample, radio resources which can be allocated to the data (service) ofthe split EPS bearer in the SeNB 12.

The information about surplus radio resources may indicate, for example,radio resources which are not used in the SeNB 12 (i.e., radio resourceswhich can be used for data transmission or the like). Examples of theradio resources may include the number of resource blocks, the number ofpackets (PDCP PDUs, PDCP SDUs, RLC PDUs, RLC SDUs, MAC PDUs (TBs),etc.), and the number of cells (i.e., the number of downlink and/oruplink carriers).

The admission control information may be information relating toadmission executed in the SeNB 12 on data (service) of the split EPSbearer (i.e., information about whether a bearer split can be accepted).More specifically, the admission control information in the SeNB 12 mayinclude at least one of the following information items (1) to (5):

(1) Information about whether or not to admit a new bearer split;(2) Information about a wait time until a new bearer split isacceptable;(3) Information about a wait time until a request for a new bearer splitis made;(4) Information about estimated (expected) throughput (data rate); and(5) Information about an estimated (expected) amount of radio resourcesto be allocated.

The information about whether or not to admit a new bearer split mayindicate, for example, whether a new bearer split is allowed in the SeNB12, or the number of new bearer splits that can be allowed in the SeNB12 (i.e., the number of EPS bearers transmitted by radio bearers (RBs)in a cell of the SeNB 12 in the case of a bearer split).

The information about a wait time until a new bearer split is acceptablemay indicate, for example, an expected minimum wait time until a bearersplit is acceptable in the SeNB 12, or a wait time until a bearer splitis acceptable.

The information about a wait time until a request for a new bearer splitis made may indicate, for example, a prohibited time during whichsending (by the MeNB 11) a request for a bearer split to the SeNB 12,i.e., sending a request for transmitting data (service) of the split EPSbearer in a cell of the SeNB 12, is prohibited.

The information about an estimated (expected) data rate (throughput) mayindicate, for example, an estimated (expected) data rate (e.g.,throughput) in the SeNB 12, or a level of a data rate (e.g., throughput)(e.g., an index value indicating one of several predetermined levels ofdata rates).

The information about an estimated (expected) amount of radio resourcesto be allocated may indicate, for example, an estimated (expected)amount of radio resources to be allocated in the SeNB 12, or a level ofan amount of radio resources (e.g., an index value indicating one ofseveral predetermined levels of the amount of radio resources). Examplesof the radio resources may include the number of resource blocks, thenumber of packets (PDCP PDUs, PDCP SDUs, RLC PDUs, RLC SDUs, MAC PDUs(TBs), etc.), and the number of cells (i.e., the number of downlinkand/or uplink carriers).

In step S35, the MeNB 11 may perform the downlink Layer 1/Layer 2control for the split EPS bearer based on the bearer split statusinformation received from the SeNB 12. As shown in FIG. 9, if necessary,the MeNB 11 may transmit, to the UE 2, an updated control message forcontrolling the downlink communication, for example, by using the RRCConnection Reconfiguration message. Further, if necessary, the MeNB 11may transmit the updated control message for controlling the downlinkcommunication to the SeNB 12.

In the downlink Layer 1/Layer 2 control in step S35, the MeNB 11 mayupdate a parameter(s) related to the LCP procedure applied to generationof downlink MAC PDUs (e.g., PBR). For example, when the average value ofradio resources allocated to the UE 2 in the SCell 120 is equal to orgreater than a predetermined value, the MeNB 11 may decrease thedownlink PBR (i.e., prioritized resource amount) for the logical channelof the split EPS bearer of the UE 2 in the PCell 110. Further, the MeNB11 may increase the downlink PBR for the non-split EPS bearer of the UE2 in the PCell 110. Accordingly, when dual connectivity involving abearer split is performed, the unfairness between downlink communicationof the split EPS bearer and downlink communication of the non-split EPSbearer can be corrected, and thus generation of MAC PDUs, transmissionpower control, and the like can be optimized so that they can beperformed as intended.

In step S36, the MeNB 11 and the SeNB 12 perform downlink communicationof the split EPS bearer with the UE 2 in accordance with the control bythe MeNB 11 in step S35. Step S36 may include downlink communication ofa non-split EPS bearer in the PCell 110.

In FIG. 9, the roles of the MeNB 11 and the SeNB 12 are interchangeable.Specifically, the MeNB 11 may report to the SeNB 12 the bearer splitstatus information related to the split EPS bearer in the MeNB 11 (PCell110). Further, the SeNB 12 may perform downlink Layer 1/Layer 2 controlin response to receiving the bearer split status information from theMeNB 11 (PCell 110).

Fourth Embodiment

In this embodiment, modified examples of the first to third embodimentsare described. A configuration example of a radio communication systemaccording to this embodiment is similar to that shown in FIG. 3. In thisembodiment, the SeNB 12 is configured to send to the MeNB 11 a requestfor a bearer split as described below. According to this configuration,dual connectivity involving a bearer split can be used more effectively.

For example, the SeNB 12 may request the MeNB 11 to increase, decrease,or update the amount of downlink data (e.g., PDCP PDU) on the split EPSbearer that is split in the MeNB 11 and is transmitted to the SeNB 12.

In another alternative, the SeNB 12 may request the MeNB 11 to adjustthe maximum transmission power allowed for the UE 2 in the PCell 110 orthe SCell 120.

In still another alternative, the SeNB 12 may request the MeNB 11 toadjust the Prioritized Bit Rate (PBR) which is applied to the logicalchannel of the split EPS bearer when the UE 2 generates the uplink MACPDUs for the PCell 110 or the SCell 120.

In yet another alternative, the SeNB 12 may request the MeNB 11 to stopthe dual connectivity involving a bearer split related to the UE 2.

These requests from the SeNB 12 to the MeNB 11 may be sent periodicallyor non-periodically (by event-triggered) according to the load of theSeNB 12 (SCell 120), or the characteristics of a physical channel (e.g.,Physical Downlink Shared Channel (PDSCH)), a transport channel (e.g.,Downlink Shared channel (DL-SCH)), or a logical channel (e.g., DedicatedTraffic channel (DTCH)).

Next, configuration examples of the MeNB 11, the SeNB 12, and the UE 2according to the first to fourth embodiments described above aredescribed. FIG. 10 is a block diagram showing a configuration example ofthe MeNB 11. A radio communication unit 111 receives an uplink signaltransmitted from the UE 2 via an antenna. A received data processingunit 113 recovers the received uplink signal. Obtained received data istransferred to other network nodes, such as Serving Gateway (S-GW) orMME of the EPC 3, or another eNB, via a communication unit 114. Forexample, uplink user data received from the UE 2 is transferred to theS-GW within the EPC 3. NAS control data contained in control datareceived from the UE 2 is transferred to the MME within the EPC 3.Further, the received data processing unit 113 receives control data tobe sent to the SeNB 12 from a communication control unit 115, and sendsthe received control data to the SeNB 12 via the communication unit 114.

A transmission data processing unit 112 receives user data addressed tothe UE 2 from the communication unit 114, and performs error correctioncoding, rate matching, interleaving, or the like, to thereby generate atransport channel. Further, the transmission data processing unit 112adds control information to a data sequence of the transport channel, tothereby generate a transmission symbol sequence. The radio communicationunit 111 generates a downlink signal by performing processing includingcarrier wave modulation based on the transmission symbol sequence,frequency conversion, and signal amplification, and transmits thegenerated downlink signal to the UE 2. The transmission data processingunit 112 receives control data to be transmitted to the UE 2 from thecommunication control unit 115, and transmits the received control datato the UE 2 via the radio communication unit 111.

The communication control unit 115 controls dual connectivity involvinga bearer split. In some implementations, the communication control unit115 may generate configuration information and control informationnecessary for dual connectivity involving a bearer split, and maytransmit the generated information to the SeNB 12 and the UE 2. Further,the communication control unit 115 may perform control of the accessstratum in response to receiving from the SeNB 12 the bearer splitstatus information (e.g., communication status information) related tothe split EPS bearer. The communication control unit 115 may send to theSeNB 12 the bearer split status information (e.g., communication statusinformation) related to the split EPS bearer to trigger the control ofthe access stratum in the SeNB 12.

FIG. 11 is a block diagram showing a configuration example of the SeNB12. The functions and operations of a radio communication unit 121, atransmission data processing unit 122, a received data processing unit123, and a communication unit 124, which are shown in FIG. 11, are thesame as those of the corresponding elements, i.e., the radiocommunication unit 111, the transmission data processing unit 112, thereceived data processing unit 113, and the communication unit 114 in theMeNB 11 shown in FIG. 10.

A communication control unit 125 of the SeNB 12 controls dualconnectivity involving a bearer split. The communication control unit125 may send to the MeNB 11 the bearer split status information (e.g.,communication status information) related to the split EPS bearer totrigger the control of the access stratum in the MeNB 11. Further, thecommunication control unit 125 may perform control of the access stratumin response to receiving from the MeNB 11 the bearer split statusinformation (e.g., communication status information) related to thesplit EPS bearer.

FIG. 12 is a block diagram showing a configuration example of the UE 2.A radio communication unit 21 is configured to support dual connectivityand to communicate simultaneously in a plurality of cells (PCell 110 andSCell 120) served by different eNBs (MeNB 11 and SeNB 12). Specifically,the radio communication unit 21 receives a downlink signal from one orboth of the MeNB 11 and the SeNB 12 via an antenna. A received dataprocessing unit 22 recovers received data from the received downlinksignal, and sends the recovered data to a data control unit 23. The datacontrol unit 23 uses the received data according to the intended use. Atransmission data processing unit 24 and the radio communication unit 21generate an uplink signal by using data for transmission supplied fromthe data control unit 23, and transmit the generated uplink signal toone or both of the MeNB 11 and the SeNB 12.

A communication control unit 25 of the UE 2 controls dual connectivityinvolving a bearer split. The communication control unit 25 performscontrol of the access stratum relating to the split EPS bearer based onan instruction from the MeNB 11 or the SeNB 12.

Other Embodiments

The communication control processes in the MeNB 11, the SeNB 12, and theUE 2 in association with dual connectivity involving a bearer split asdescribed in the first to fourth embodiments may be implemented by asemiconductor processing device including an Application SpecificIntegrated Circuit (ASIC). These processes may be implemented by causinga computer system including at least one processor (e.g., amicroprocessor, a Micro Processing Unit (MPU), or a Digital SignalProcessor (DSP)) to execute a program. Specifically, one or moreprograms including instructions for causing the computer system toperform algorithms described above with reference to sequence diagramsand the like may be created, and the program(s) may be supplied to acomputer.

The program(s) can be stored and provided to a computer using any typeof non-transitory computer readable media. Non-transitory computerreadable media include any type of tangible storage media. Examples ofnon-transitory computer readable media include magnetic storage media(such as flexible disks, magnetic tapes, hard disk drives, etc.),optical magnetic storage media (e.g. magneto-optical disks). CompactDisc Read Only Memory (CD-ROM), CD-R, CD-R/W, and semiconductor memories(such as mask ROM, Programmable ROM (PROM), Erasable PROM (EPROM), flashROM, Random Access Memory (RAM), etc.). The program(s) may be providedto a computer using any type of transitory computer readable media.Examples of transitory computer readable media include electric signals,optical signals, and electromagnetic waves. Transitory computer readablemedia can provide the program to a computer via a wired communicationline, such as electric wires and optical fibers, or a wirelesscommunication line.

In the first to fourth embodiments, the LTE system is mainly described.However, as described above, these embodiments may be applied to radiocommunication systems other than the LTE system, such as a 3GPP UMTS, a3GPP2 CDMA2000 system (1×RTT, HRPD), a GSM/GPRS system, or a WiMAXsystem.

Further, the above embodiments are only illustrative of the applicationof the technical idea obtained by the present inventor. That is, thetechnical idea is not limited only to the above embodiments and can bemodified in various ways as a matter of course.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2013-227473, filed on Oct. 31, 2013, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   1 EVOLVED UTRAN (E-UTRAN)-   2 USER EQUIPMENT (UE)-   3 EVOLVED PACKET CORE (EPC)-   11 MASTER eNodeB (MeNB)-   12 SECONDARY eNodeB (SeNB)-   25 COMMUNICATION CONTROL UNIT-   110 PRIMARY CELL (PCell)-   120 SECONDARY CELL (SCell)-   115 COMMUNICATION CONTROL UNIT-   125 COMMUNICATION CONTROL UNIT

1-43. (canceled)
 44. A first radio station, comprising: a memory storinginstructions; and at least one processor configured to process theinstructions to: serve a first cell; control a radio terminal to causethe radio terminal to perform dual connectivity using the first cell anda second cell served by a second radio station; receive a first PowerHeadroom on the first cell and a second Power Headroom on the secondcell from at least the radio terminal; and transmit the first PowerHeadroom on the first cell and the second Power Headroom on the secondcell to at least the second radio station.
 45. A second radio station,comprising: a memory storing instructions; and at least one processorconfigured to process the instructions to: serve a second cell;communicate with a radio terminal configured to perform dualconnectivity using a first cell served by a first radio station and thesecond cell; and receive a first Power Headroom on the first cell and asecond Power Headroom from at least the first radio station.
 46. A radiocommunication system comprising: a first radio station configured toserve a first cell; a second radio station configured to serve a secondcell; and a radio terminal configured to perform dual connectivity usingthe first cell and the second cell, wherein the radio terminal isconfigured to transmit a first Power Headroom on the first cell and asecond Power Headroom on the second cell to at least the first radiostation, and the first radio station is configured to transmit the firstPower Headroom on the first cell and the second Power Headroom on thesecond cell to at least the second radio station.
 47. A control methodfor a first radio station, comprising: serving a first cell; controllinga radio terminal to cause the radio terminal to perform dualconnectivity using the first cell and a second cell served by a secondradio station; receiving a first Power Headroom on the first cell and asecond Power Headroom on the second cell from at least the radioterminal; and transmitting the first Power Headroom on the first celland the second Power Headroom on the second cell to at least the secondradio station.
 48. A control method for a second radio station,comprising: serving a second cell; communicating with a radio terminalconfigured to perform dual connectivity using the first cell and asecond cell served by a second radio station; and receiving a firstPower Headroom on the first cell and a second Power Headroom on thesecond cell from at least the first radio station.